At present, single cells must be connected in series to form battery packs in energy storage power plants, electric vehicles and other applications. Since there exists differences in the battery capacity and performance caused by production process, and the differences will be expanded during the charging and discharging process of the battery pack, the risks of excessive charging and discharging will increase. At the same time, the battery pack capacity utilization rate is decreased, which will accelerate the damage to the battery in a vicious cycle. Therefore, in order to further improve the consistency of the battery pack, enhance the battery pack capacity utilization rate, extend the service life of battery pack, it is required to perform necessary maintenance for the battery pack, and maintain charging for batteries in the battery pack with low surplus capacity, and maintain discharging for batteries in the battery pack with high surplus capacity.
Currently, there are mainly maintenance methods, and generally it is divided into two types: one is voltage-based maintenance method, and the other is capacity-based maintenance method.
The voltage-based maintenance method is to perform maintenance discharging for batteries with high voltage and to perform maintenance charging for batteries with low voltage through various charging maintenance and discharging maintenance ways by maintaining battery pack's inconsistency via the voltage difference of single cells collected. This method is easy to control and widely applied. It usually includes the current shunt method and energy transfer method. For the current shunt method, generally the current of single cell is adjusted through resistors, and the maintenance current is controlled by switches to carry out discharging maintenance of single cells with excessively high voltage. For the energy transfer method, generally the energy is transferred from high-voltage cells to low-voltage cells through various media, to improve the consistency of battery pack voltage. The energy transfer media can be capacitors, inductors, transformers, etc.
The most fundamental purpose of maintenance is to balance the remaining capacity between batteries. The voltage-based maintenance mode can reflect the capacity characteristics, but can not accurately describe the status of remaining capacity of battery, thus the excessive balance phenomenon may appear, to worsen the consistency between battery pack. Internal chemical materials of lithium battery may result in this phenomenon. The polarization effect of lithium battery may cause the deviation of battery voltage from the balance value when the current flows through the battery; since it is unable to achieve consistency in the lithium battery production process, the remaining capacity may be different under the same voltage. Thus, the capacity-based balancing method can make up the above shortcomings, and provide safe and effective balance during the full cycle of lithium battery, extending its service life. The maintenance module is directly used to maintain discharge of battery with excessive remaining capacity and to maintain charging of battery with low remaining capacity. However, the capacity-based maintenance method needs to accurately estimate SOC of single cells; and the maintenance reliability will greatly reduced if the accuracy of SOC can not guaranteed.